The present invention is directed toward baseband processors for use in communications systems, for example, wireless communications systems and, more particularly, toward a baseband processor having look-ahead parameter estimation capabilities for improved channel tracking performance.
Communication via wireless networks is becoming prevalent in today""s society. Additional radio spectrum is being allotted for commercial use, and cellular phones are becoming increasingly commonplace. Due to the variety of different frequency bands being used, the need for dual-band phones, operational in each of the different frequency bands, has arisen. For example, in the United States, wireless phone service is offered in both the cellular (approx. 800 MHz) and PCS (Personal Communications Services) (approx. 1900 MHz) frequency bands. Wireless phone service providers generally have a mixture of cellular and PCS licenses. Thus, to offer seamless service across the country, customers of wireless phone service require dual-band phones.
In addition, there is currently an evolution from analog to digital communications. Digital speech is represented by a series of bits, which are modulated and transmitted from a base station to a phone, and vice versa. The phone demodulates the received waveform to recover the bits as originally transmitted, and the recovered bits are then converted back into speech. Further, growing data services, such as E-mail, Internet access, etc., require digital communications.
Many types of digital communications systems are currently available. For example, FDMA (Frequency-Division-Multiple-Access) systems divide the radio spectrum into a plurality of radio channels corresponding to different carrier frequencies. TDMA (Time-Division-Multiple-Access) systems further divide the carrier frequencies into time slots. D-AMPS (Digital Advanced Mobile Phone System), PDC (Pacific Digital Cellular), and GSM (Global System for Mobile communications) are examples of digital TDMA cellular systems. Alternatively, if the radio channel is wide enough, multiple users can use the same channel using spread spectrum techniques and CDMA (Code-Division-Multiple-Access). IS-95 and J-STD-008 are examples of wireless systems incorporating CDMA standards.
Regardless of the modulation or multiple access method used, a communications system must provide good quality, such as good speech quality, which is critical for customer satisfaction. To provide the necessary high quality at the receiver end, advanced demodulation techniques are generally employed, such as coherent demodulation with channel estimation and, if necessary, Automatic Frequency Correction (AFC).
A transmitted radio signal passes through a transmission medium, often referred to as a channel. Conventional receivers use channel estimation to compensate for the effects of the channel on the received signal. Channel estimation is typically performed by a baseband processor at the receiver and involves estimating channel coefficients associated with the received signal. The channel coefficients represent modifications occurring to the received signal during transmission. In many wireless applications, the quantities to be estimated change with time, such that the channel coefficient changes need to be tracked. This is also called adaptive parameter estimation. Tracking is typically performed with decision feedback, using past symbol detections to assist in updating the channel coefficients. In certain receiver designs, such as Maximum Likelihood Sequence Estimation (MLSE) receivers, decision feedback channel estimation requires a delay to obtain reliable detected symbol values. This introduces a delay in the channel tracking processing, leading to poor performance when the channel is changing rapidly.
For better channel estimation, it is desirable to have not only information about past and current symbol detections, but also information after the symbol period of interest. One solution is to employ periodically placed pilot symbols in the signal at the transmitter end. At the receiver end, the channel is estimated over the pilot symbols and interpolated in between. However, pilot symbols introduce additional overhead, reducing information symbol energy and/or the number of information symbols that can be transmitted. An alternative is to employ a noncoherent receiver which does not require explicit estimation of the channel, such as a noncoherent MLSE receiver, or a noncoherent MAP (Maximum A-Posteriori Probability) symbol-by-symbol detector. However, these receivers are much more complex and costly than standard coherent receivers.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
A method is provided of detecting received symbol values in a communications system. In one form, the method generally includes the steps of receiving data samples including a sequence of symbol values, developing sets of future hypothetical symbol values, developing sets of parameter estimates, one set for each of the sets of future hypothetical symbol values, based on the received data samples and the sets of future hypothetical symbol values, developing a plurality of metrics, one each for the sets of parameter estimates, based on the sets of parameter estimates and the received data samples, and developing detected symbol values based on the plurality of metrics, the detected symbol values related to the received data samples.
In another form, the method generally includes the steps of receiving data samples including a sequence of symbol values, hypothesizing combinations of symbol values, developing, subsequent to the hypothesizing step, parameter estimates associated with each combination of hypothesized symbol values based on the received data samples and the hypothesized symbol values, developing, subsequent to the step of developing parameter estimates, metrics associated with each parameter estimate based on the received data samples, the hypothesized symbol values and the parameter estimates, and developing, subsequent to the step of developing metrics, detected symbol values based on the metrics, the detected symbol values related to the received data samples.
In yet another form, the method generally includes the steps of receiving data samples including sequences of symbol values, developing first sets of hypothetical symbol values for a first set of symbol periods, developing second sets of hypothetical symbol values for a second set of symbol periods, wherein the second set of symbol periods includes at least one symbol period later in time than the symbol periods in the first set of symbol periods, developing parameter estimates based on the received data samples and the second sets of hypothetical symbol values, developing metrics, more than one each for the first sets of hypothetical symbol values, based on the received data samples, the parameter estimates and the first sets of hypothetical symbol values, and developing detected symbol values based on the metrics, the detected symbol values related to the received data samples.
In still another form, the method generally includes the steps of receiving data samples including sequences of symbol values, developing a plurality of parameter estimates based on the received data samples, developing, subsequent to the step of developing parameter estimates, a plurality of metrics based on the developed parameter estimates and the received data samples, and developing, subsequent to the step of developing metrics, detected symbol values based on the plurality of metrics, the detected symbol values related to the received data samples.
A baseband processor is also provided for use in a receiver receiving data samples transmitted across the channel. In one form, the baseband processor generally includes a hypothesize unit developing sets of hypothetical symbol values, a parameter estimator developing parameter estimates based on the received data samples and the sets of hypothetical symbol values, and a sequence estimator receiving the developed parameter estimates, the sets of hypothetical symbol values and a delayed version of the received data samples and developing a sequence of detected symbol values related to the received data samples.
In another form, the baseband processor generally includes a first hypothesize unit developing first sets of hypothetical symbol values, a second hypothesize unit developing second sets of hypothetical symbol values different from the first sets, a parameter estimator receiving the received data samples and the first and second sets of hypothetical symbol values and developing parameter estimates based thereon, and a sequence estimator receiving the developed parameter estimates, the first sets of hypothetical symbol values and a delayed version of the received data samples and developing a sequence of detected symbol values related to the received data samples.
It is an object of the present invention to provide a baseband processor having look-ahead parameter estimation capabilities.
It is a further object of the present invention to provide a baseband processor capable of performing channel estimation on hypotheses of future symbols, as well as hypotheses of current and past symbols.
It is yet a further object of the present invention to provide baseband processor capable of performing parameter estimation that exploits information about the channel before, during and after the symbol period of interest without adding additional overhead signaling.
It is still a further object of the present invention to provide a baseband processor capable of updating channel estimates prior to updating branch and path metrics for demodulation.
Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims.